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| United States Patent |
5,562,721
|
|
Marchlinski
, et al.
|
October 8, 1996
|
Method of using endocardial impedance for assessing tissue heating
during ablation
Abstract
A method of locating infarcted myocardial tissue in a beating heart
includes the step of inserting an impedance measuring tip of a catheter
into the chamber of the beating heart, particularly the left or right
ventricle, and measuring the impedance of the endocardium at various
locations within the chamber of the beating heart. The values measured are
compared to impedance values with a predetermined range of values to
identify an infarcted area of myocardium and distinguish such area from
normal myocardium. The measurements are also compared to a range of values
for an infarction border zone. In accordance with the invention, the
infarction border zone may be located. The infarction border zone is a
significant source of arrhythmia, and particularly of ventricular
tachycardia. Further, in accordance with the methods of the present
invention, the risk of arrhythmia in a beating heart may be substantially
reduced or eliminated by ablating endocardium within the infarction border
zone utilizing the same catheter tip. Impedance measurements may also be
utilized to assess the adequacy of the electrode-tissue contact,
particularly in a fluid filled body organ or cavity. Further, the
effectiveness of the ablation of the tissue may be determined by
determining the degree of heating of the tissue by measuring the change in
impedance in the area of ablation.
| Inventors:
|
Marchlinski; Francis E. (Bala Cynwyd, PA);
Schwartzman; David S (Philadelphia, PA);
Mirotznik; Mark S. (Silver Spring, MD);
Foster; Kenneth R. (Haverford, PA);
Gottlieb; Charles D. (Wynnewood, PA);
Chang; Isaac (Philadelphia, PA)
|
| Assignee:
|
University of Pennsylvania (Philadelphia, PA);
Philadelphia Heart Institute (Philadelphia, PA);
Catholic University of America (Washington, DC)
|
| Appl. No.:
|
438326 |
| Filed:
|
May 10, 1995 |
| Current U.S. Class: |
607/99; 607/122 |
| Intern'l Class: |
A61B 017/39; A61N 001/05 |
| Field of Search: |
128/654,672,695 R,734,693
607/41,99
|
References Cited
U.S. Patent Documents
| 4380237 | Apr., 1983 | Newbower | 128/693.
|
| 4674518 | Jun., 1987 | Salo | 128/695.
|
| 4852580 | Aug., 1989 | Wood | 128/693.
|
| 4911174 | Mar., 1990 | Pederson et al. | 128/695.
|
| 5058583 | Oct., 1991 | Geddes et al. | 128/419.
|
| 5092339 | Mar., 1992 | Geddes et al. | 128/692.
|
| 5195968 | Mar., 1993 | Lundquist et al. | 604/95.
|
| 5197467 | Mar., 1993 | Stenhaus et al. | 128/419.
|
| 5224475 | Jul., 1993 | Berg et al. | 128/419.
|
| 5239999 | Aug., 1993 | Imran | 128/642.
|
| 5246008 | Sep., 1993 | Mueller | 128/695.
|
| 5282840 | Feb., 1994 | Hudrlik | 607/28.
|
| 5334193 | Aug., 1994 | Nardella | 606/41.
|
| 5341807 | Aug., 1994 | Nardella | 128/642.
|
Other References
Myocardial Electrical Impedance Mapping of Ischemic Sheep Hearts and
Healing Aneurysms-Circulation, vol. 87, pp. 188-207 (1993).
|
Primary Examiner: Sykes; Angela D.
Assistant Examiner: Huang; Stephen
Attorney, Agent or Firm: Petock, Esq.; Michael F.
Parent Case Text
This application is a division of application Ser. No. 08/188,514, filed
Jan. 28, 1994, which is now pending.
Claims
We claim:
1. A method for assessing effectiveness of tissue ablution, comprising the
steps of:
measuring impedance of tissue at an ablation area at normal body
temperature to establish a baseline impedance value;
monitoring impedance of the tissue in an area being ablated immediately
after application of ablation energy;
comparing the base line impedance value to the monitored tissue impedance
to determine an absolute impedance measurement; and
correlating an amount of tissue heating to the absolute impedance
measurement, a substantially linear decrease in impedance corresponding to
rising tissue temperature and effective tissue ablation.
2. A method in accordance with claim 1, wherein the previously recited
impedance measuring steps are carried out utilizing an electrode mounted
on a tip of a catheter.
3. A method in accordance with claim 2, wherein said method is utilized
within a beating heart to determine adequacy of endocardial tissue
ablation in a beating heart.
4. A method for assessing effectiveness of tissue ablation, comprising the
steps
inserting into a beating heart a catheter having an electrode with a design
which comes directly into contact with endocardium in such a manner as to
minimize contact of a measuring electrode with blood in the beating heart;
measuring impedance of endocardium at an ablation area at normal body
temperature to establish a baseline impedance value:
monitoring impedance of the endocardium in an area being ablated
immediately after application of ablation energy;
comparing the baseline impedance value to the monitored endocardium
impedance to determine an absolute impedance measurement; and
correlating an amount of endocardium heating to the absolute impedance
measurement, a substantially linear decrease in impedance corresponding to
rising tissue temperature and effective tissue ablation.
5. A method for assessing effectiveness of tissue ablation in accordance
with claim 4 wherein said catheter electrode is placed in it entirety
directly in contact with the endocardium during said impedance measuring
steps.
6. A method for assessing effectiveness of tissue ablation in accordance
with claim 4 wherein said steps of measuring impedance are performed using
a sharp tipped catheter electrode to achieve exclusive tissue contact by
piercing endocardium.
Description
FIELD OF THE INVENTION
The present invention relates to methods of using measurements of
endocardial impedance for the determination of arrhythmiogenic sites for
catheter ablation, assessing catheter-tissue contact and methods of
confirming tissue ablation during and after energy delivery. More
particularly, the present invention relates to methods of determining an
infarction border zone in a beating, post-myocardial infarcted heart,
using impedance measurements to insure adequate catheter-tissue contact
and measuring the impedance as a confirmation of heating of the tissue
during the ablation process, the later two being useful on various tissues
and organs and not limited to cardiac applications.
BACKGROUND OF THE INVENTION
It has been known for some time that one of the long term sequelae of a
myocardial infarction is the generation of arrhythmias, such as
tachycardia which may result in fibrillation of the heart and sudden
death. Accordingly, for some time, efforts have been directed at reducing
the risk of such arrhythmias. For years, attempts have been made to reduce
the risk of arrhythmia by pharmacological treatment.
More recently, a surgical approach to the eradication of tissue which
generates ventricular tachycardia has been utilized which renders the
target endocardium and sub-endocardium electrically inert or surgically
removes it. This surgical procedure has been demonstrated to be highly
effective, however perioperative mortality is high due to left ventricular
failure, and only a small percentage of patients with ventricular
tachycardia are candidates for this procedure.
Most recently, attempts to eradicate arrhythmic tissue have included the
application of radiofrequency energy via an electrode mounted on a
catheter tip, known as "catheter ablation". For example, see U.S. Pat. No.
5,239,999--Imran.
There are significant problems with the catheter ablation process as
previously practiced, including the inability to judge adequate contact
between the ablating electrode and the target endocardium. Another problem
is the inability to locate appropriate targets for ablation. Still another
problem is the inability to determine when the radiofrequency energy
applied via an electrode mounted on a catheter successfully ablates the
tissue intended to be ablated.
In the past, techniques to localize the endocardial origin of ventricular
tachycardia in the setting of chronic myocardial infarction have utilized
only electrogram characteristics. These techniques have included sinus
rhythm mapping, activation mapping, pace-mapping and entrainment mapping.
These techniques have poor specificity for localization of the site of
origin of ventricular tachycardia. In addition, to properly perform some
of these techniques, long periods of sustained tachycardia are necessary,
often placing a significant hemodynamic burden on the patient.
SUMMARY OF THE INVENTION
The present invention is directed to a method of locating infarcted
myocardial tissue in a beating heart which includes the step of inserting
an impedance measuring tip of a catheter into a chamber of a beating
heart, measuring the impedance of the endocardium at various locations
within the chamber of the beating heart and comparing the measured
impedance values with a predetermined range of values and/or assessing
differences in impedance ranges to identify an infarcted area of
myocardium and distinguish such area from normal myocardium.
It has been found that there is a two fold crease in impedance measured on
the endocardium of infarcted tissue as contrasted to normal endocardium.
Ranges of values may be tabulated and impedance measurements compared with
these values. Alternatively, measurements may be taken on various surfaces
of the endocardium and compared with each other to determine infarcted
areas as well as border zones between infarcted endocardium and normal
endocardium. The border zone between normal and infarcted endocardium,
particularly in the ventricles, is often a source of the generation of an
arrhythmia such as ventricular tachycardia.
In accordance with the present invention, a method of reducing the risk of
arrhythmia in a beating heart utilizes the step of inserting a tip of a
catheter into a chamber of a beating heart wherein the tip is adapted for
both impedance measurement and ablation. The impedance of the endocardium
at various locations within the chamber of the beating heart is measured
using the tip of the catheter in the measuring mode of operation. Once the
border zone between normal and infarcted endocardium, referred to herein
as the infarction border zone is located, sufficient energy is applied to
the tip of the catheter to ablate endocardium in the infarction border
zone.
Further in accordance with the present invention, a method is provided of
assessing the adequacy of electrode-tissue contact in a fluid filled organ
which includes the steps of inserting an impedance measuring electrode
mounted on a catheter into a desired portion of a fluid filled organ and
determining whether the electrode is in contact with the organ tissue
based on the impedance value. In a presently preferred embodiment, the
method is utilized to assess the adequacy of electrode contact with the
endocardium in a catheter ablation process by first measuring the
impedance value when the electrode is in blood, such as in the aorta
outside of the heart to determine a base line value and detecting the
change in impedance when the electrode comes in contact with the
endocardium.
Further, in accordance with the present invention, a method of assessing
the effectiveness of tissue ablation utilizes the steps of measuring the
impedance of tissue at or around the area of tissue to be ablated at body
temperature and measuring the impedance of the tissue in the area being
ablated during the application of ablation energy inorder to assess the
degree of heating of the tissue. It has been found that tissue impedance
declines substantially proportionally to tissue temperature during the
ablation process. Accordingly, in cardiac catheter ablation, the
effectiveness of the ablation of the endocardium may be monitored to
insure, by measuring the change in impedance, that sufficient heating of
the endocardium has taken place to insure tissue ablation adequate to
eliminate the arrhythmiogenic site.
The terms impedance or electrical impedance as utilized herein are intended
in their broadest sense, that is including the resistive component and/or
inductive reactance and/or capacitive reactance, including the condition
wherein the capacitive and inductive reactances may cancel or are
non-existent leaving only the resistive component as the impedance. In a
co-pending application filed the same day as this application, application
Ser. No. 08/138,142, filed by some of the same applicants herein and
entitled Systems and Methods for Examining the Electrical Characteristics
of Cardiac Tissue, the term "E-Characteristic" has been utilized in
connection with such impedance and resistance values.
BRIEF DESCRIPTION OF THE DRAWINGS
For the purpose of illustrating the invention, there are shown in the
drawings forms which are presently preferred; it being understood,
however, that this invention is not limited to the precise arrangements
and instrumentalities shown.
FIG. 1 is a view in perspective of a catheter tip which may be utilized in
practicing the method of the present invention.
FIG. 2 is a graph of impedance values illustrative of the principles of the
method of the present invention.
FIG. 3 is an elevation view of an alternate embodiment of a catheter tip
which may be utilized in practicing the method of the present invention.
FIG. 4 is a graph of decreasing impedance values with increasing
temperature of tissue being ablated in accordance with the method of the
present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A myocardial infraction incurs when an area of the heart is deprived of
blood and therefore oxygen. After the death of cardiac tissue, the tissue
is replaced with scar tissue. At the border between the scar tissue and
the normal endocardium, there is a mixture of normal cells and scar
tissue. Normal endocardium is comprised mainly of cardiac muscle cells.
Infarcted myocardium is comprised mainly of protein strands without cells.
The infarction border zone represents a gradual transition in tissue
protein strand content. It is the protein strand content which causes the
differences in impedance between normal, infarction and infarction border
zone. Infarction border zone is an area of endocardium which often
generates arrhythmias such as ventricular tachycardia.
It is the infarction border zone which is desired to be located for the
purposes of ablation.
In accordance with the method of the present invention, it has been found
that areas of normal tissue, infarcted tissue and areas immediately
adjacent infarcted tissue, referred to herein as the infarction border
zone may be identified or located within a beating heart by measuring the
impedance of the endocardium at various points. It has been found in
accordance with the method of the present invention that the three types
of myocardial tissue namely normal, infarcted and infarction border zone
tissue, present distinct ranges of impedance when measured in accordance
with the methods of the present invention. It has been found that there is
substantially a two fold increase in the impedance values of normal tissue
as contrasted to infarcted tissue. Further, there is approximately a one
fold increase in the infarction border zone as compared to infarcted
tissue.
Although not limited to the locating of arrhythmias arising in the
ventricles, the present invention is particularly useful for locating the
source of such ventricular tachycardias. Further, in accordance with the
present invention, once such potential source of arrhythmia, namely the
infarction border zone, has been located, it may be ablated in accordance
with the methods of the present invention utilizing the same catheter tip
by the application of suitable amounts of RF energy, thereby reducing or
eliminating the risk of arrhythmias such as ventricular tachycardias.
In accordance with the present invention, a catheter tip such as that shown
in FIG. 1 may be utilized, although various other types of catheter tips
may be utilized in practicing the method of the present invention. In
accordance with the method of the present invention, it has been found to
be advantageous to have a catheter tip design which comes directly into
contact with the endocardium in such a manner as to reduce contact of the
measuring tip with the blood in the beating heart, thereby enabling direct
measurement of the impedance of the endocardium, as contrasted to the
impedance of blood which is of a significantly lower value. An important
concept in the technique of utilizing electrodes for assessing the
adequacy of tissue contact is that the electrode be configured in such a
way as to permit contact solely with the tissue of interest. The impedance
value obtained by the electrode will then reflect the impedance of the
contacted tissue only. Partial contact with tissue will render an
ambiguous impedance value. It is further preferred that the catheter tip
have heat dissipating design features for more accurate readings.
One suitable electrode for a catheter tip is illustrated in FIG. 1 wherein
the tip is provided with a disc shape 10 which is provided with a
predetermined significant depth which enhances heat dissipation. In the
one example illustrated in FIG. 1, the disc shaped catheter may preferably
have dimensions of a diameter 12 of 2.33 mm and a depth of 0.25 mm.
However, it is understood that other dimensions of catheter tips may be
utilized in practicing the method of the present invention. The catheter
shaft 14 is preferably fitted with a recording electrode 16 which acts as
a second electrode for recording purposes, such as recording local
electrograms. The catheter utilized herein may be provided with one or
more other electrodes so that various electrograms may be measured.
Electrograms may also be measured with the impedance measurement
electrode. Adequate recording of electrograms does not require sole
contact of an electrode with the endocardial area of interest.
Referring to FIG. 3, there is shown another embodiment of an electrode 30
mounted on a catheter tip 32 which may be utilized in practicing the
present invention. Catheter tip shown in FIG. 3 is provided with a sharp
tip 34 and is relatively long, being 2 millimeters long and of a
relatively small diameter, approximately 0.2 millimeters. The electrode 30
may be utilized to achieve exclusive tissue contact at odd angles. It will
be apparent to those skilled in the art that various dimensions and
modifications may be made to the electrodes for the catheter tips in
accordance with the present invention.
As will be described more fully hereinafter, contact of such an electrode
with the endocardium will provide a much higher impedance value than
contact with the surrounding blood, thereby providing a means of
endocardial contact assessment. The impedance difference between the
endocardium and the blood depends on the surface area of the electrode,
smaller surface areas record larger differences.
In practicing the method of the present invention, the catheter tip is used
for both measuring impedance and ablating tissue. Tissue is ablated by the
application of RF energy in suitable amounts, as is well known in the
electrode catheterization art. The catheter may be guided to any chamber
or area of the heart. In practicing the method of the present invention,
the measuring electrode may be mounted in any way that allows direct
contact with the endocardium. For example, left ventricular endocardial
mapping may be performed by percutaneous insertion of the catheter into
the femoral artery using the Seldinger technique, then retrograde passage
of the catheter via the aorta and into the ventricle after crossing the
aortic valve. Of course, access to any endocardial site in either atria or
either ventricle may be achieved. In addition, mapping of the epicardium
may be performed by thoracotomy and direct application of the electrode to
the heart. The guidance of catheters into the heart, often using
fluoroscopy, is known as cardiac catheterization, and is well known to
those skilled in the art and need not be described here in detail.
Once the catheter tip is located in the appropriate chamber of the heart,
the catheter tip may be manipulated to engage the myocardial area of
interest with an appropriate contact pressure to achieve sole tissue
contact to allow accurate measurement of the impedance at various
locations. Utilization of the rendered impedance value may be performed
using normal values based on research on subjects with normal hearts,
and/or by using the impedance values measured in clearly normal areas of
the heart of the subject undergoing investigation. By comparing values in
these ways, areas of infarction or infarction bordering endocardium may be
discerned. In this manner, suitable sites for application of energy for
the performance of catheter ablation may be determined. Adequacy of
electrode-tissue contact may be assessed as described hereinafter.
Ranges of impedance may be predetermined for normal endocardium, densely
infarcted endocardium and infarction border zone endocardium as
illustrated in FIG. 2. FIG. 2 illustrates specific values for normal,
infarcted and infarction border zone endocardium measured in a significant
number of sheep measured at 550 kHz. As illustrated in FIG. 2 at 20,
normal endocardium tissue has an impedance range as illustrated with a
mean in the neighborhood of 350 ohms. Endocardium in the infarction border
zone has an impedance range as illustrated at 22 with a mean value of
about 250 ohms. Densely infarcted endocardium has a tissue range as
illustrated at 24 with a mean value of about 100 ohms. Generally,
impedance values measured on infarcted endocardium were approximately 25%
of those in normal endocardium and impedance values from infarction border
areas were approximately 60-70% of those in normal endocardium. Although
specific values may vary by electrode tip and the like, easily discernable
differences in impedance are reproducibly achieved between normal,
infarction border zone and infarcted endocardium.
In this manner, by comparing values measured at various points within the
endocardium, infarcted tissue may be determined. Furthermore, normal
tissue may be determined and most importantly, the infarction border zone
may be determined. The infarction border zone has been determined to be a
frequent source of arrhythmias, particularly of ventricular tachycardia.
Accordingly, by use of the method of the present invention, sites for
ablation within a beating heart may be determined utilizing cardiac
catheterization techniques, and such sources of potential arrhythmias may
be ablated, thereby reducing or eliminating the risk of arrhythmia, and
particularly of ventricular tachycardia.
The method of determining the tissue to be ablated described herein is not
limited to cardiac catheterization ablation for purposes of eliminating
arrhythmiogenic sites. The method of the present invention may be utilized
to measure impedances within various organs or body cavities to detect
differences in tissue impedance to enable a determination as to a site to
be ablated. For example, a tumor in the liver or bladder may be located by
the detection of different impedance values between normal and tumor
tissue, thereby enabling the ablation of such tumor tissue. The method of
the present invention may be utilized to detect various different
conditions in tissue which are accompanied by a change in impedance
values.
In accordance with the method of the present invention, assessment of
adequate contact between the electrode on the catheter tip and tissue in a
fluid filled cavity or organ may be assessed. In a preferred application
of practicing the method of the present invention, adequacy of
electrode-endocardium contact may be assessed in a blood filled pumping
heart. It has been found that there is significant impedance difference
between the impedance of blood and endocardium tissue. This difference is
very pronounced between blood and normal endocardium tissue. However, even
with respect to infarcted endocardium, the impedance of the blood is
characteristically measured at ranges approximately 25% less than the
values achieved for infarcted tissue. This 25% change in impedance may be
used to make the determination or alternatively specific values may be
obtained for each patient by making an impedance measurement in the blood,
preferably in a vessel, such as the aorta, outside of the heart, and
another impedance measurement when the electrode is in direct stable
placement with the endocardium as determined visually using fluoroscopy.
Accordingly, by noting or monitoring the values of measured impedance, an
assessment may be made as to whether there is adequate contact between the
catheter tip electrode and the endocardium.
When radiofrequency energy is passed through an electrode which is in
contact with the inside of the heart (endocardium), the volume of the
endocardium which is in contact with the electrode will heat. If the
tissue gets hot enough, it will die. If the tissue which is killed was
part of an aberrant electrical circuit, such as those which cause cardiac
arrhythmias, the arrhythmia will be cured. If the contact between the
electrode and the endocardium is poor, the radiofrequency energy is
quickly dissipated in the blood passing through the heart, and no
significant accumulation of heat is achieved in the endocardium.
Accordingly, the foregoing method of assessing adequate contact between
the electrode and the endocardium is of great importance.
This method of assessing the adequacy of electrode-tissue contact may be
utilized in any body cavity or organ which has a direct blood or other
fluid (e.g. cerebrospinal fluid) interface.
Further, even if the contact is adequate, it is important to determine that
the RF energy applied via the electrode mounted on the catheter tip is
causing sufficient heating of the endocardium to allow for successful
ablation. Should the endocardium be heated only to a lower temperature at
which it can survive and return to normal function, if this tissue were a
critical part of the propagation path of an arrhythmia, it is possible
that the arrhythmia which was thought to be permanently eradicated during
a given catheter ablation procedure may be only temporarily damaged.
Unexpected recurrence of arrhythmias may lead to dangerous symptoms,
including death in some cases and to morbidity, expense and risk of
repeated hospitalization and further procedures. Accordingly, it is
important to be able to determine during and immediately after the
ablation process that the target endocardium to be ablated was
sufficiently heated so as to sufficiently ablate the particular area of
endocardium to prevent further arrhythmia generation.
In accordance with the present invention, it has been found that actual
heating of the myocardium during the application of RF energy is
associated with a reproducible linear change in the impedance of the
myocardium. See FIG. 4. Since heating by the application of RF energy
causes the ablation, it is important that the degree of heating of the
endocardium or other tissue to be ablated be determined. Sufficient
heating of the endocardium is capable of curing an arrhythmia resulting
from that tissue. A temperature of approximately 50 degrees centigrade is
required to successfully ablate myocardium. It is possible for endocardium
which is heated to lower temperatures to survive and return to normal
function.
In order to assess or monitor the heating of the endocardium, the impedance
of the endocardium may be monitored during the application of the RF
energy and/or immediately after the application of the RF energy. These
impedance measurements are compared to a base line impedance value
established by measuring the impedance in or around the area to be ablated
at normal body temperature, preferably, but not necessarily, immediately
before the application of the RF energy for ablation. Once a consistent
measurement of impedance is obtained in the base line state,
radiofrequency energy or other ablation energy is applied. As the
myocardium located at the surface of the electrode heats, local impedance
changes, and such changes may be continuously measured via the energy
delivering electrode.
In practicing the method of assessing the degree of heating during the
ablation process, as well as in the electrode-tissue contact assessment,
it is preferable to use a catheter electrode such as that shown in FIG. 1
although other types of electrodes may be utilized in practicing the
method the present invention so long as the design criteria outlined above
are taken into account.
FIG. 4 is illustrative of the relationship between increasing tissue
temperature and decreasing impedance. FIG. 4 illustrates data obtained
using the electrode of FIG. 1 on the epicardium of live pigs, measured in
unipolar fashion at 550 kilohertz. A substantially linear decrease in
impedance was shown in association with rising tissue temperature at the
electrode surface induced by the application of RF energy. This pattern
was highly reproducible. Accordingly, the tissue impedance monitoring
provides reliable information of tissue temperature at the site of energy
application via the electrode. It is believed that this is the only
unequivocal evidence of actual tissue heating.
Although the data is reproducible and values may be established in which
absolute impedance measurement correlate with a predetermined amount of
tissue heating, in the preferred method of practicing the invention, each
patient's impedance values at body temperature, before application of
energy may be used to establish the base line. In this way, each patient
acts as his/her own standard of reference, from which the degree of tissue
heating may be judged from the amount of tissue impedance decrease with
each energy application.
Numerous variations may be made in practicing the methods of the present
invention without departing from the spirit or scope of the present
invention. Various frequencies may be utilized in the measuring process
ranging from 1 kilohertz to 1 megahertz. It has been found that by using
frequencies at less than 100 kilohertz, better resolution of impedance
values is obtained for demonstrating tissue heating. In the preferred
method of practicing the invention, impedance has been measured at
frequencies at which RF energy is applied through commercially available
devices, namely in the 500 to 750 kilohertz range. Although larger
differences in ranges of impedance values between normal, infarction
border and infarcted tissues are found at lower frequencies, frequencies
utilized may be up into the megahertz range. However, preferably the
frequency used is less than 1,000 kHz. In a preferred method of practicing
the invention to date, impedance measurements have been made at 550 kHz.
At lower frequency ranges, particularly in the 1 kHz range, although the
largest differences between ranges of impedance values for normal,
infarcted and infarction border zone tissue are observed, electrode
polarization artifacts may present a serious problem. However, as referred
to above, various other types of catheter tips may be utilized, including
a catheter tip having four spaced electrodes mounted in an insulative
base. A fixed current may be passed through endocardium between the two
outer electrodes and the voltage developed in the endocardium between the
two inner electrodes may be measured. The impedance may be readily
calculated by the equipment as the ratio of voltage to current. Typically,
a small subthreshold current of approximately 15 micro-amperes alternating
current may be utilized for this purpose.
It is further noted that the methods described and illustrated herein are
not limited to use in the myocardium. The impedance measuring method to
determine differences in tissue, the method of determining whether the
electrode is in contact with the tissue and the method of assessing
adequate ablation of undesired tissue may be used in various applications
in the body, including, but not limited to, ablation of tumors, cancerous
or benign, or the like.
In view of the above, the present invention may be embodied in other
specific forms without departing from the spirit or essential attributes
thereof and, accordingly, reference should be made to the appended claims,
rather than to the foregoing specification as indicating the scope of the
invention.
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